1

Laboratory

Evaluation of

Two Optical

Instruments for

Real-Time

Particulate

Monitoring of

Smoke

Laboratory

Evaluation of

Two Optical

Instruments for

Real-Time

Particulate

Monitoring of

Smoke

United States

Department of

Agriculture

Forest Service

Technology &

Development

Program

2500 Watershed

February 1999

9925-2806-MTDC
2

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(voice and TDD).

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Director, Office of Civil Rights, Room 326-W, Whitten

Building, 14th and Independence Avenue, SW,

Washington, DC 20250-9410 or call (202) 720-5964

(voice or TDD). USDA is an equal opportunity provider

and employer.

Laboratory

Evaluation of

Two Optical

Instruments

for Real-Time

Particulate

Monitoring of

Smoke

Laboratory

Evaluation of

Two Optical

Instruments

for Real-Time

Particulate

Monitoring of

Smoke

Andy Trent

Project Leader

Harold Thistle

Program Leader

Rich Fisher

Air Resource Management, Washington Office

Ronald Babbitt

Electronics Engineer, Fire Sciences Laboratory

Andria Holland-Sears

Hydrologist, White River National Forest

USDA Forest Service

Technology and Development Program

Missoula, Montana

8E82F54—Evaluate Two Nephelometers

February 1999
3

Table of Contents

Introduction_______________________________________________________ 1

Instruments _______________________________________________________ 2

Methods __________________________________________________________ 4

Results and Discussion ___________________________________________ 5

Conclusions _____________________________________________________ 10

Additional Tests __________________________________________________ 10

ii
4

Introduction

As land management agencies

continue aggressive wildland

 burning programs, the effects of

increased airborne particulate matter

become of more concern to land

managers. Real-time monitoring of

airborne particulates is critical to the land

manager’s ability to protect human health,

public safety, and visibility. Land

managers need reliable methods to

measure particulate concentrations at the

time of the fire to help make operational

decisions. Gravimetric techniques require

that filters collect the particulate matter.

The filters are later weighed at special

facilities routinely used in air quality

applications. Instruments that infer

particulate concentration by measuring

optical properties of the atmosphere (light

scattering, absorption, or extinction) are

more practical and useful for near realtime measurement.

In March and July of 1998, the Missoula

Technology and Development Center

(MTDC) and the Fire Sciences

Laboratory (FSL) evaluated two real-time

optical instruments that estimate

particulate concentrations by comparing

them with two gravimetric devices. The

March laboratory tests were conducted to

determine whether the optical and

gravimetric instruments showed

significant differences when measuring

smoke particles produced from burning

wood under controlled conditions in an

indoor chamber. Analysis of the data from

these tests showed considerable

differences between the two

measurement techniques. Further

laboratory tests in July gathered data to

develop a correction curve for the optical

instruments and to determine if there were

significant differ-ences in the optical

properties of smoke based on the type of

fuel generating it.

All results and conclusions were

based only on laboratory tests.

Further testing of the instruments

in a field environment will be

conducted to validate the laboratory results.

This evaluation was funded by the USDA

Forest Service’s Watershed, Soil, and Air

Program.

1
5

Instruments

2

Several Forest Service Regions

 have already purchased optical

 instruments to measure airborne

particulate levels. Two types of these

instruments are the MIE DataRAM

portable real-time aerosol monitoring

system and the Radiance Research

M903 nephelometer. Both instruments

measure the extinction coefficient of light

due to light backscattering (Bscat).

Aerosol levels are commonly reported in

mass concentration, usually expressed in

micrograms per cubic meter (ug/m3

). The

mass concentration is estimated from

Bscat by applying a conversion factor.

The DataRAM (Figure 1) is a compact,

self-contained instrument that internally

estimates mass concentration from the

measured Bscat (particulate

measurement ranges from 0.1 ug/m3

 to

400,000 ug/m3

). It continuously displays

the current and time-weighted average

mass concentration while logging up to

10,000 data points. Optionally, the

DataRAM can be configured with either a

PM-2.5 or PM-10 impactor head to

prevent particles larger than 2.5 or 10

microns, respectively, from entering the

optical chamber. In these tests, the PM

2.5 impactor head was used. For custom

calibrations, particulate can be collected

on a filter located in the base of the

instrument. An in-line heater may also be

installed for monitoring in conditions of

high humidity (typically above 70%

relative humidity) or fog. The tubular

heater is designed to raise the

temperature of the sampled air stream to

evaporate liquid water from airborne

particles or to eliminate fog droplets.

The M903 Radiance Research portable

nephelometer (Figure 2) is a lightweight,

low-power instrument designed for

portable operation as well as general

environmental monitoring. It measures

and displays

Bscat, not mass

concentration like

the DataRam.

Mass

concentrations

must be calculated

by hand from the

Bscat readings.

The instrument

has a particulate

measurement range from approximately 1

to 1000 ug/m3

 when converted from

Bscat. The instrument has an internal data

logger that will store scattering coefficient

averages and the operating parameters

that are used to estimate the Bscat. The

stored data can be retrieved using a

personal computer through an RS232

port. Different averaging times and log

intervals may be set. The instrument can

store approximately 2 weeks of 5-minute

averages.

We investigated a third instrument, the

DustLite, manufactured by Rupprecht and

Patachnick. The DustLite estimates mass

concentration much like the MIE

DataRAM, but the purchase price is a

Figure 1–The MIE DataRam. Figure 2–The Radiance Research nephelometer.
63

fraction of the DataRAM’s price. However,

after an initial evaluation, we concluded

that this instrument would not be

satisfactory for Forest Service field use.

Gravimetric filter samples of 2.5 um and

smaller were collected concurrently with

the optical instruments to determine actual

particulate levels. For the March tests,

two different instruments were used to

gather the gravimetric samples. One was

an Environmental Protection Agency

(EPA) Reference Method PM 2.5 sampler

manufactured by Rupprecht & Patachnick

Co., Inc. (R&P). IML Air Science in

Sheridan, WY, weighed the 47-mm

diameter Teflon filters used by this

instrument. The second instrument was a

PM 2.5 sampler developed by the Fire

Sciences Laboratory (FSL) for conducting

airborne smoke studies. This instrument’s

37-mm diameter filters were weighed in a

special environmentally controlled facility

at the Fire Sciences Laboratory.

Concentrations of particulate in

micrograms per cubic meter were

calculated from flow and gravimetric data.

For the July series of tests, only the

RMFSL filter system was available.
7

Methods

Figure 3

47%

4 11/16 x 3 1/8

10 x 6 3/4

Print to Outside Edge of Borders

DO NOT Print Borders

Figure 4

52%

3 1/2 x 5 3/8

6 3/4 x 8 1/2

Print to Outside Edge of Borders

DO NOT Print Borders

Figure 3–Instrument layout on the smoke platform in the burn chamber.

Figure 4–An example of the firebeds of pine needles used to produce

smoke in the laboratory burn chamber.

4

The laboratory study was

conducted in the Fire Sciences

Laboratory’s large (131,000-ft3

)

combustion chamber. All instruments

were calibrated and operated on a

smoke-sampling platform 55 feet above

the chamber floor (Figure 3). The

experiments were performed at ambient

conditions inside the closed chamber

(70 to 90° F and 30 to 50% relative

humidity). Small beds of flaming and

smoldering ponderosa pine needles on

the chamber floor (Figure 4) generated

smoke for the majority of tests. Smoke

for the remainder of the tests was

generated by burning a small amount of

duff. The instruments were close to one

another so they would be sampling

approximately the same air. Fans helped

mix the smoke in the chamber.

A total of 66 tests were completed in

March and July. The duration varied for

each test. The higher concentration tests

were shorter to prevent the gravimetric

filters from becoming clogged. The lower

concentration tests were longer to allow

enough particulate to accumulate on the

filters for accurate measurements. The

average duration of the tests was about

1 hour.
8

Results and Discussion

Figure 5–Estimated mass concentration readings of DataRAMs and converted mass concentration readings from the Radiance Research

Nephelometer for Test No. 7 in March 1998.

Particulate Monitor Test - All Instruments

Test No. 7 

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

13:48:3013:50:0013:51:3013:53:0013:54:3013:56:0013:57:3013:59:0014:00:3014:02:0014:03:3014:05:0014:06:3014:08:0014:09:3014:11:0014:12:3014:14:0014:15:3014:17:0014:18:3014:20:0014:21:3014:23:0014:24:3014:26:0014:27:3014:29:0014:30:3014:32:0014:33:3014:35:0014:36:30

Time

Instrument Concentration (

u g/m3

)

Radiance (Fire Lab)

Radiance (MTDC #1)

Radiance (MTDC #2)

Region 5 DataRAM

Region 2 DataRAM

5

The estimated mass concentration

of the DataRAM and the converted

Bscat mass concentrations from

the Radiance Research nephelometers

give similar estimates of real-time

particulate concentrations. Figure 5

shows the particulate concentration

estimates of two DataRAMs and three

Radiance Research nephelometers

during Test No. 7 in March 1998. For this

test, the temperature was 68° F and the

relative humidity was 25%. The average

value for each instrument was within +/-

6% of the mean value for all instruments.

The particulate concentrations measured

by the optical instruments were

significantly higher than those measured

by the gravimetric instruments for all

concentration ranges and all instruments.

Figures 6, 7, and 8 show the respective

nephelometer concentrations plotted

against the gravimetric concentrations.

Concentration estimates for the optical

instruments were made using conversion

factors supplied by the manufacturer.

Based on the best-fit lines, the Region 2

DataRAM, MTDC DataRAM, and the

Radiance Research nephelometer

showed PM 2.5 concentrations 2.1, 1.9,

and 1.9 times higher, respectively, than

those measured by gravimetric devices.

The readings were nearly linear with a

correlation coefficient (R2

) of 0.97 and

higher.

Test results were compared when

different materials (pine needles and duff)

were burned. There was no significant

difference in the instrument readings

between the tests. Figure 9 shows the

MTDC DataRam mass concentration

readings for both pine needle and duff

smoke plotted against the gravimetric

concentrations.
9

Best Fit Line for Region 2 DataRAM

All Data - March & July 98

y = 2.1605x

R2

 = 0.9722

0

200

400

600

800

1000

1200

0 200 400 600 800 1000 1200

Filter Concentration (u g/m3)

R2 DataRAM Concentration (

u g/m3

)

Region 2 DataRAM

Linear (Region 2 DataRAM)

Figure 6–Best-fit line for Region 2’s DataRam based on the instrument’s estimated mass concentrations and results from the gravimetric sampler.

6
10

Best Fit Line for MTDC DataRAM

All Data - July 98

y = 1.9342x

R2

 = 0.9784

0

100

200

300

400

500

600

700

800

900

0 100 200 300 400 500 600 700 800 900

Filter Concentration (u g/m3)

MTDC DataRAM Concentration (

u g/m3

)

MTDC DataRAM

Linear (MTDC DataRAM)

Figure 7–Best-fit line for MTDC’s DataRam based on the instrument’s estimated mass concentrations and results from the gravimetric sampler.

7
11

Best Fit Line for MTDC Radiance Research Nephelometer

All Data - March & July 98

y = 1.9014x

R2

 = 0.9715

0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400 500 600 700 800 900 1000

Filter Concentration (u g/m3)

Radiance Research Concentration (

u g/m3

)

Radiance Research

Linear (Radiance Research)

Figure 8–Best-fit line for the Radiance Research nephelometer based on the instrument’s converted mass concentrations and results from the

gravimetric sampler.

8
12

Figure 9–Comparison of the DataRam’s estimated mass concentration readings for smoke generated from pine needles and duff compared to results

from the gravimetric sampler.

Comparison of DataRam Readings for 

Smoke Generated from Pine Needles and Duff

0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400 500 600 700 800 900 1000

Gravimetric Concentration (u g/m3)

Instrument Concentration (

u g/m3

)

MTDC DataRam (Pine Needles)

MTDC DataRam (Duff)

9
13

Conclusions

10

The MIE DataRAM and Radiance

Research nephelometer both

overestimated mass

concentration of biomass smoke.

Caution is advised when using optical

instruments to determine mass

concentrations of airborne smoke

particulate. More accurate estimates of

particulate concentration of biomass

smoke can be achieved with the instruments tested if a correction factor of 0.53

(1/1.9) is applied to the optical reading

(DataRAM concentration * 0.53). Since

the correction factor is linear, the DataRAM

can be programmed by the user to

automatically output the corrected value.

For the Radiance Research instrument,

multiply the Bscat conversion factor suggested by the vendor by 0.53 when converting from Bscat to mass concentration.

For this laboratory study, burning different

biomass materials did not significantly

affect the concentration estimates.

The effects of other common atmospheric scattering aerosols, such as

sulfates, nitrates, and soil material were

not evaluated. Our suggested correction

factor may not be appropriate when the

ambient aerosol budget is dominated by

these nonbiomass aerosols or even

when it includes some sizeable fraction

of them.

When the ambient relative

humidity is higher than 70%,

optical instruments will have

even greater bias estimating particulate

mass concentrations than during dry

atmospheric conditions. A high priority

should be given to further investigating

nephelometer operation in smoky, highhumidity environments. From this work,

Additional Tests

appropriate correction curves and/or

more effective dryers may be developed.

A series of field tests should be

conducted using optical instruments and

gravimetric instruments. Setting up the

instruments adjacent to a relatively large

prescribed burn would provide a realistic

comparison.

Further tests should be conducted with

different fuel types (such as green

branches and needles, or deciduous

trees) and with different pollutant types

(particularly sulfates, nitrates, and soil

material) to further identify instrument

characteristics.
i

About the Authors

Ronald Babbitt has bachelor’s degrees in forestry and electrical engineering. He joined the Forest Service in 1975 and has worked in areas of forest

utilization, forest engineering, and fire research. He now works with the Fire Chemistry Research Project at the Fire Sciences Laboratory in Missoula,

MT.

Rich Fisher has bachelor’s degrees in life science (U.S. Air Force Academy) and in meteorology (North Carolina State University). He has master’s

degrees in earth science (Colorado State University) and in international and strategic studies (Naval War College). He is a certified consulting

meteorologist and is Reserve Assistant to the Director of Weather for the U.S. Air Force in the Pentagon.

Harold Thistle received a Ph.D. in plant science specializing in forest meteorology from the University of Connecticut in 1988. He is

certified by the American Meteorological Society as a Certified Consulting Meteorologist (CCM), and worked as a consultant in private

industry before joining MTDC in 1992. He served as the Center’s Program Leader for Forest Health Protection until 1998, developing

modeling techniques that accurately describe transport of pesticides in the atmospheric surface layer and evaluating meteorological

instrument systems for environmental monitoring. He now works with the Forest Health Technology Enterprise Team in Morgantown, WV.

Andy Trent is a Project Engineer at MTDC. He received his bachelor’s degree in mechanical engineering from Montana State University in 1989.

He came to MTDC in 1996 and works on projects for the Nursery and Reforestation, Forest Health Protection, and Watershed, Soil, and Air

Programs.

Andria Holland-Sears received her bachelor’s degree in watershed science from Utah State University in 1981. Before moving to Colorado

in 1994, she worked extensively as an Air Resource Specialist for the Forest Service in the Pacific Southwest Region (5) out of Lake Tahoe.

Now she works as a hydrologist on the White River National Forest. She also provides technical support related to smoke management and

air quality to the Forest Service Regional Office in Denver.

Additional single copies of this document may be ordered from:

USDA Forest Service

Missoula Technology and Development Center

5785 Hwy 10 W

Missoula, MT 59808

Phone: (406) 329-3900

Fax: (406) 329-3719

E-mail: wo_mtdc_pubs@fs.fed.us

For further technical information, please contact Harold Thistle or Andy Trent at the address above.

Harold Thistle

Phone: (304) 285-1574

E-mail: hthistle@fs.fed.us

Andy Trent

Phone: (406) 329-3912

Fax: (406) 329-3719

E-mail: atrent@fs.fed.us